In current processes used to produce a heterojunction bipolar transistor (HBT), a base/collector mesa etch is designed to stop on top of or within a subcollector contact layer. For HBTs grown on an indium-phosphide (InP) substrate, the contact layer is typically composed of gallium-indium-arsenic(GaInAs). With contact layers composed of GaInAs, specific contact resistivies of 1×10−7 Ohm-centimeter2 are typically obtained with non-alloyed low Ohmic metal composed of, for example titanium (Ti), platinum (Pt) and/or gold (Au).
When a base collector mesa etch involving a final aqueous acid etch—such as a solution of water (H2O) and hydrochloric acid (HCl) at a 1 to x mixture where x is less than or equal to 1, or such as a solution of phosphoric acid (H3PO4) and HCl at a 1 to x mixture for all x—is employed, then the InP over the collector region is removed selectively and a GaInAs subcollector contact layer is not etched. See for example, N. Matine, M. W. Dvorak, J. L. Pelouard, F. Pardo, and C. R. Bolognesi, “Fabrication and characterization of InP HBTs with emitter edges parallel to [001] and [010] crystal orientations,” Jap. J. Appl. Phys. Part 1, vol. 38, no. 2B, pp. 1200-1203, 1999. Such a contact layer may be kept very thin (e.g., 50 Angstroms) and still function as an etch stop because of the excellent selectivity in HCl based etches.
When a dry etch is used to perform the base collector mesa etch, selectivities are far inferior or nonexistent than when a final aqueous acid etch is used. The dry etch, therefore, requires that much thicker GaInAs contact layers be used. For example, when a dry etch is used, the GaInAs contact layers are typically required to be many hundreds of Angstroms to a few thousand Angstroms thick. However, the thermal conductivity of GaInAs is approximately 0.05 Watts per centimeter° Kelvin (W/cm° K) compared to approximately 0.68 W/cm° K for InP. This makes thermal management much more difficult with the presence of GaInAs in the subcollector. See, for example, E. Sano, K. Kunshima, H. Nakajima, and S. Yamahata, High-speed, low-power lightwave communication ICs using InP/InGaAs double-heterojunction bipolar transistors,” IEICE Trans. Electron., vol. E82-C, no. 11, pp. 2000-2006, 1999. For this reason a GaInAs contact has not been compatible with a dry etch base/collector mesa etch.
Furthermore, the common approach of using sintered Ohmic contacts (e.g. gold-germanium-nickel (AuGeNi)) is not compatible with HBT fabrication, because typical sintering cycle temperatures of approximately 400° C. (typical for gold-germanium (AuGe) eutectic alloying) are incompatible with already present emitter and base Ohmic contacts.